Germanium photodiode



July 16, 1957 B. SCHWARTZ ETAL GERMANIUM PHOTODIODE Filed Sept. 1, 1955BERNARD SCHWARTZ ilnited States Fatent GERMANIUNI PHOTODIODE BernardSchwartz, Watertown, and Patrick W. Malone-y, Beverly, Mass, assignorsto Sylvania Electric Products Inc, a corporation of MassachusettsApplication September 1, 1953, Serial No. 377,918

8 Claims. (Cl. 317-234) This invention relates to the structure andmethods of manufacture of electrical devices having a hermeticallysealed envelope. One aspect relates to means and methods forobtaining-rigid mounting of the parts within an envelope and tohermetically sealing the parts therewithin.

It is an object of the invention to mount within an envelope theassembled components of such electrical devices as contact rectifiers,photodiodes and the like in such manner that they will be able towithstand severe shock and vibration.

It is a further object to seal heat sensitive electrical devices in anenvelope with minimum hazard of injury to the devices from the heat.

A still further object is to form a fused glass seal in an envelopeenclosing a semiconductor.

An additional object is to evacuate the envelopes of electrical devices,especially miniature devices, and seal such envelopes, while soevacuated, in an effective manner.

Sometimes electrical devices are made exceedingly small so that, as inthe case of manufacture of the crystal photodiodes here exemplified asembodiments of our invention, the envelope may be less than /8 of aninch long and less than A; of an inch in diameter. In such cases, wherethe envelope is of glass, or the like, the leads from the electrodesexit from the envelope through a very thin insulating wall, and the wallis very apt to crack or chip away due to any intended or accidentalbending of the exterior portion of the lead-in wires of the tube. It istherefore another object of this invention to provide for a constructionwhich shall avoid this possibility of breakage while yet providing for agood hermetic seal between the leads and the envelope.

It is desirable to seal these devices against admission of water vaporand other deleterious gases; and it is further desirable to evacuatetheir envelopes. Because of the exceedingly small size of the tube thereis great difiiculty in exhausting the same. Where parts are so small, toprovide the envelope with a discrete exhaust tubulation for the exhaustport of a conventional vacuum tube exhausting and sealing machineinvolves practical difficulties. It is therefore still another object ofthis invention to provide a new and effective means and method forexhausting exceedingly small envelopes, and to heat-seal such evacuatedenvelopes despite the sensitivity of the enclosed components to heat.

Furthermore, the ordinary methods of sealing the glass with gas flameshaving pencil point sharp tips while eflicacious in sealing the portionsof the envelope together yet may give rise to damage to electrodeswithin tubes of the size here dealt with since heating with gas does notconfine the heat to the selected area desired to be heated as much aswould be desired. It is therefore yet a further object of the inventionto provide for a new mode of eifecting sealing of the tubes.

These and other objects will be apparent after reading the followingspecification and claims when taken in conjunction with the accompanyingdrawing in which Fig. 1 is a cross sectional view through a cylindricalceramic plug utilized in the construction of an illustrative germaniumphotodiode embodying features of this invention.

Fig. 2 is an enlarged view partly in section showing the plug with leadsthreaded therethrough and sealed thereto, with a semiconductor junctionphotodiode ready to be joined to the leads.

Fig. 3 is an enlarged view showing the photodiode in assembledrelationship with the envelope in the process of being sealed to theplug.

Fig. 4 illustrates the completed photodiode.

Fig. 5 is a still greater enlarged sectional view of a completedphotodiode along the line 5-5 of Fig. 4, and

Figs. 6 and 7 illustrate alternative means and methods of sealing theenvelope in fabricating the device of Fig. 4.

It should be understood that although the invention has been illustratedspecifically as applied to a photodiode, it will be apparent, afterreading the specification, that portions of the invention are applicableto many forms of electronic tubes and are not necessarily restricted tophotodiode construction.

Referring to the drawings more in detail, there is disclosed acylindrical plug 1 made of a suitable ceramic material as steatite inwhich are formed a number of bores 3. In the ease of the photodiodechosen for illustrative purposes, there are two bores. Inserted throughthese bores are conventional leads 5 such as dumet, these leads beingsealed to the ceramic by fusing low melting glass frit or suitable glazearound the leads where they enter or leave the plug, as indicated at 7and 9. In the case of the form of invention disclosed in Figs. 1 to 4,the glass at the top of the plug not only embraces the leads 5 butextends down about the periphery of the ceramic plug as indicated at 11.The lower ends of one of the leads 5 is slightly curved away from theother lead, as at 17 and suitably secured to this curved end, by Weldingor otherwise, is a fine nickel wire 19 as an extension of its lead 5.Likewise, mounted on the other lead, is a nickel strip 21 as anextension of its lead 5, the lower end of which is bent backwardly, asat 23. Mounted on the lower face of the bent portion is an N-typegermanium die 25, surmounted by a germanium-indium alloy button 27. Thenickel wire is joined to the button. In use, the junction between theindium and the germanium die is exposed to a source of light, as will besubsequently explained more in detail. The fabrication of such junctionphotodiode involving meticulous heat-treatment, etching and othertreatments, is well known and need not be elaborated here; but thejunction is readily damaged by excessive heating, and it is desirablefor stability and long life that the unit be enclosed in a sealedenvelope.

After the electrode parts have been thus assembled, the bulb 29 isslipped over the unit, into telescopic relation to the ceramic plug 1,until the end of the bulb 30 is engaged by end 32 of wire 19 and thebulb is fused to the plug by means of the low-melting glass 13 throughsuitable application of heat, as from a flame 31.

The bent back portion 23, in the completed tube, see Fig. 5, has itsrectangular corners 33 resting against the inner Wall of the bulb 29while the nickel wire 19 has an outwardly bowed portion 35 thrustingagainst the opposite Wall of the bulb. Thus the junction and its leadsare restrained against lateral movement through three point contact ofthe electrodes with the lateral Walls of the bulb; Further by reason ofthe mounting of the electrodes on the lead-in wires 5, there is provideda mount for the electrodes which resists any displacement of partsendwise of the bulb due to shock or vibration.

The bulb 29 is provided with a thickened lens portion 30 for focusinglight onto the junction of the indium alloy button and the germanium.The rest of the bulb is advantageously coated with some opaque substance39, as a black paint, to prevent stray light from hitting the sensitiveportion of the diode. Apart from its photosensitivity, the junction isan excellent rectifier; and where so used, the coating can be extendedover the end of bulb 29, to guard against stray effects due to lightchanges.

By virtue of the inherently tough ceramic plug, the leads may bedeformed Without special danger of cracking, as would be entailed in anall-glass end. Also, the leads can be placed very close to each otherwithout danger of their drifting together during the head-sealing of theenvelope, as would frequently occur were the sealed end formed of softglass.

The structure of Fig. 4 may be both evacuated and sealed as indicated inFig. 6. The glazed ceramic plug of Fig. 3 is controlled carefully so asto be slightly smaller in diameter than the inner diameter of bulb 41,that is purposely much longer than bulb 29. The entire plug carrying theleads and the rectifying junction is dropped into the bulb and the plugis fused throughout its length to the glass bulb that is heatedexternally as by flame 43. This is effected while the free end of thebulb is held in a rotating chuck 45. Prior to the sealing operation justdescribed, the envelope is evacuated by suitable vacuum pumps, similarto the techniques employed to evacuate radio tubes during theirmanufacture. Subsequent to the sealing operation, the skirt 46 of thebulb 41 may be cut off immediately above the ceramic plug. The cut-offedge may be flame polished if desired.

In Fig. 7 there is illustrated another mode of joining the plug andenvelope. at the locus of fusion of the ceramic parts, with possibilityof heat spreading beyond the desired region and thereby damaging thesemiconductor junction, the heat of fusion is obtained by electricinduction. About the area to be heated, there is painted a ring orstripe of electrically conductive material 49, as silver paint, orsilver oxide reduced to metallic silver by firing, in a preliminarytreatment in a reducing-atmosphere furnace. The assembly of parts, as inFig. 3, is inserted in a furnace Whose wall 51 is an insulator, asquartz. To effect fusion of the glass, the painted portion of theenvelope is inductively coupled to high-frequency energized coil 53,whereupon the metallic ring or stripe 49 acts as a shorted secondary andheats up sufficiently to soften and fuse the adjacent glass frit orglaze on the ceramic. Thus a sealing of parts is accomplished. The lowerportion of quartz Wall 51 is of reduced diameter to hold bulb 29 uprightand keep band 49 spaced from Wall 51 during heating operations. As inFig. 6, the bulb is evacuated before sealing is effected, by suitablevacuum pumps; also suitable getters in the vacuum line may be used. Theimperfect fit of glazed plug 1 within bulb 29, before the heating andsoftening of the low-melting glass 13 is started, provides a passage forevacuation of the bulb. Because this passage is constricted, the periodof evacuation may be somewhat longer than would be the case Where aseparate exhaust tubulation is formed on the envelope. But the addedbulb-heating involved in tipping off such tubulation and the added sizeof the resulting unit make such exhaust tubulation undesirable, sincethe units as shown may be packed one against the other in a row or in arectangular matrix for sensing punch cards.

Having thus described our invention, what we claim as new is:

1. An electrical device comprising an envelope, mul- Instead ofutilizing a gas flame 4 tiple leads in said envelope braced against thewalls thereof, one of said leads carrying a rectifying junction, saidrectifying junction being in contact with a second of said of leads andengaging said second lead at one point, only, intermediate the lengththereof.

2. An electrode tube comprising an envelope, a pair of electrodes insaid envelope with their upper ends in spaced apart relation, one ofsaid electrodes carrying a rectifying junction, said rectifying junctionlying in contact with the other of said electrodes toward the lower endsof the electrodes, each of said electrodes having portions bearingagainst the walls of the envelope while in said contacting relation toinhibit separating movement of the electrodes.

3. An electrical device comprising an envelope having a cylindricalwall, a pair of leads within said envelope having their lower ends inspaced apart relation, one of said leads having an end bent toward thecylindrical wall, with corners of the so bent end bearing against thecylindrical wall at two points, a rectifying junction carried by saidbent end and engaged by the other one of said leads, said other leadbeing bowed outwardly intermediate of its ends into bearing engagementwith the cylindrical wall of the envelope.

4. An electrical device comprising an envelope having elongated walls, apair of lead-ins rigidly sealed in one end of said envelope, a contactrectifying device carried by at least one of said lead-ins in saidenvelope at the opposite end thereof, each of said lead-ins havingportions bearing against the elongated walls of the envelope.

5. An electrical device comprising an envelope and lead-ins therein, aninsulating plug at the otherwise open end of said envelope through whichsaid lead-ins extend,

. a conductive device in said envelope and connected to said lead-ins, aglass seal about an end face of said plug and extending about the sidesof the plug, and said envelope comprising a portion sealed to saidextending glass.

6. An electrical device comprising an envelope and lead-ins therein, aplug at the bottom of said envelope through which said lead-ins extend,a conducting device in said envelope and connected to said lead-ins, aglass seal about the bottom and top of the plug, said glass sealextending about the entire periphery of the plug and said envelopecomprising a portion fused to the glass about the plug to form a sealedenvelope.

7. An electrical device comprising an evacuated envelope and lead-inwires extending through an end thereof, a ceramic plug at the bottom ofsaid envelope through which said lead-in wires extend, a rectifyingdevice within said envelope remote from said plug and carried by saidlead-in wires, a low-melting glass seal extending over about the insideend of said plug and extending about the sides of the plug, and saidenvelope having a portion telescoped over the plug, said portion beingfused to the envelope by the low-melting glass seal.

8. The method of fabricating a hermetically sealed and evacuatedrectifying device, including the steps of forming a glass tube with oneend closed and with the opposite end open, applying an encirclingconductive film to the exterior of the tube adjacent its open end,forming a subassembly including a rectifying junction on leads sealed inand extending through a ceramic plug and spaced from said junctionsufficiently to afford thermal isolation, coating the ends of said plugand the sides thereof contiguous with one end with a low-melting glass,inserting said rectifying junction into the tube to a position adjacentthe closed end thereof with the plug extending into said open endadjacent said encircling conductive filrn, inserting the tube and theinserted subassembly into a chamber having insulating walls about andspaced from said encircling film, evacuating the chamber and said tube,and inductively heating said film from the exterior of the chamber toheat-seal said plug to the open end of said tube.

(References on following page) 6 References Cited in the file of thispatent 2,595,475 McLaughlin May 6, 1952 UNITED STATES PATENTS 2, 6 ,528Stelmak Dec. 29, 1953 2, 03,856 P 1. 8, 1955 1,720,311 Becket July 9,1929 7 at 3 Mar 1,842,525 Heintz Jan. 2?, 19;; 5 -FOREIGN PATENTS {3 3 13 251,706 Great Britain May 11, 1926 108,056 Australia July 21, 19372,223,031 Edwards Nov. 26, 1940

1. AN ELECTICAL DEVICE COMPRISING AN ENVELOPE, MULTIPLE LEADS IN SAIDENVELOPE BRACED AGAINST THE WALLS THEREOF, ONE OF SAID LEADS CARRYING ARECTIFYING JUNCTION, SAID RECTIFYING JUNCTION BEING IN CONTACT WITH ASECOND OF SAID OF LEADS AND ENGAGING SAID SECOND LEAD AT ONE POINT,ONLY,INTERMEDIATE THE LENGTH THEREOF. REOF.